Magnetic Properties of Encapsulated Nanoparticles in Nitrogen-Doped Multiwalled Cabon Nanotubes Embedded in SiO _x Matrices

We report the production, characterization, thermal transformations (400–1000 °C), and magnetic properties of nanoparticles encapsulated in nitrogen-doped multiwall carbon nanotubes (CN _x -MWNT), which were embedded in silicon oxide (SiON _x ) matrices via sol–gel techniques. The vapor chemical deposition (CVD) method with ferrocene-benzelamine mixtures was used to synthesize Fe and Fe₃C nanoparticles inside CN _x -MWNTs. Composites consisting of CN _x -MWNTs (filler) and SiON _x (matrix) were fabricated and thermally treated to different temperatures and exposure times (t). All samples were characterized using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), thermogravimetic analysis (TGA), and magnetometry (vibrating sample). We found that upon thermal treatment, the ferromagnetic nanoparticles modify their morphology, composition and aspect ratio, thus resulting in drastic changes in the magnetic and structural properties. In particular, as produced encapsulated nanoparticles mainly consisting of Fe and Fe₃C phases were thermally modified into magnetite (Fe₃O₄. We have also observed that the hysteresis loops are very sensitive to the thermal treatment of the sample. Thus we can control the magnetic properties of the samples using thermal treatments.